1. Field of the Invention
This invention generally relates to a gas laser system and in particular to a ballast resistor device for use with a gas laser tube. More specifically, the present invention relates to improvements in electrical insulation and heat dissipation of a laser ballast resistor device.
2. Description of the Prior Art
A gas laser system is typically used as a light source in laser printers and video disc recorders. It is well known that the gas laser must be operated at constant current so as to obtain a stable laser output with less noise. Since the gas laser typically possesses the characteristic of negative resistance, it is common practice to provide a ballast resistor as connected in series with the gas laser tube in order to cancel this effect. If a gas laser having such a negative resistance characteristic were driven by a constant current source, an oscillation would be induced in the current source thereby making the operation unstable.
For example, a typical He-Ne laser of a few mW output class possesses the characteristics: discharging onset voltage of 10 kV or less, discharging voltage of 1 kV to 2 kV, discharging current of 5-5.5 mA and negative resistance of 30-70 k-ohms. A ballast resistor to be used with such a gas laser is required to have the following characteristics. (1) resistance value: 100 k-ohms (must be larger than the negative resistance of laser) (2) power consumption: 4-5 W (3,025 W if the discharging current is 5.5 mA and the resistance of ballast resistor is 100 k-ohms) (3) breakdown voltage: 10 kV or more (must be larger than discharging onset voltage)
In addition, such a ballast resistor must be disposed close to the anode of the gas laser tube so as to make the effects of parasitic capacitance of leads as small as possible and to stabilize the power source. Provision of the ballast resistor close to the anode of the laser tube then requires that the ballast resistor be high in breakdown voltage as well as high in heat dissipation. However, although prior art laser ballast resistors are electrically insulated sufficiently, they are not as high as desired in heat dissipation so that they have often been found to be damaged by heat. Stated more in detail in this respect with reference to FIG. 1, a typical prior art gas laser system includes a gas laser tube 3 provided with an anode pin 1 and a cathode pin 2 at each end as illustrated and a ballast resistor device 4 mounted on the peripheral surface of the gas laser tube 3 as located closer to the anode pin 1 as sealed in a glass tube 5. The anode pin 1, cathode pin 2 and ballast resistor 4 are connected in series, and such a series loop is further connected to a laser power source 6 typically comprised of DC/DC and AC/DC converters. In this manner, since the prior art ballast resistor device is fixedly mounted on the laser tube as sealed in the glass tube, a sufficient electrical insulation may be attained but it is relatively poor in the characteristic of heat dissipation thereby providing the probability of being damaged by heat accumulation.